1. Field of the Invention
The present invention relates to processes for removing dissolved silica from geothermal liquids, particularly to prevent silica scaling in geothermal brine power plants and associated reinjection equipment.
2. Description of the Prior Art
Appreciable quantities of naturally occurring steam and/or hot aqueous liquid (geothermal fluids) can be found in many subterranean regions of the world. In general, these are regions where the thermal gradient of the earth's crust is abnormally high, as in areas of volcanic activity and along the Pacific Ocean rim.
Geothermal steam and hot water or brine, where readily available and advantageously located, have been used in several countries for direct heating, industrial processes and therapeutic purposes. However, a potentially much more important use for such geothermal fluids is for generation of electrical power, the use of which is less site restricted.
General techniques are known whereby geothermal steam and hot geothermal water or brine can be used for generating electric power. For example, geothermal steam after treatment to remove particulate matter and such gases as hydrogen sulfide can be directly used to drive steam turbine/generators, and high temperature geothermal water and brine may be flashed to extract steam which is then used to drive a steam turbine/generator. More moderate temperature geothermal water and brines may be used in binary heat recovery systems to vaporize a low boiling point working fluid, the resulting vapor being used to drive a gas turbine/generator.
However, in actual practice serious difficulties have typically been associated with obtaining the large quantities of more commonly found geothermal water (brine) necessary to operate competitively sized geothermal power plants, and particularly in handling and disposing of the usually heavily contaminated and frequently highly saline geothermal brines. As a result, the actual or projected costs of electricity generated from geothermal brines have generally not been competitive with cost of more conventionally produced power. These difficulties and high costs associated with geothermal brine electric power generation, coupled with heretofore abundant supplies of cheap hydrocarbon fuels, have tended to retard development of geothermal brine power plants.
Many of these serious problems encountered have been associated with disposal of the large volumes of brine effluent from even modest sized geothermal power plants. This geothermal brine effluent, which may typically be discharged at continuous rates of several hundred thousand pounds per hour, usually contains excessive levels of dissolved salts. Hence, the effluent usually cannot be further used for crop irrigation or the like and cannot, without further treatment, be safely discharged into water supplies.
Most geothermal brine effluent is consequently reinjected into the ground, usually so as to return to the geothermal reservoir from which it was extracted. Such reinjection also helps prevent ground subsidence over the reservoir, which might otherwise be caused by fluid extraction; further, reinjection to the reservoir tends to increase the amount of energy extractable therefrom.
Some of the most serious of these disposal problems have been associated with this reinjection of geothermal brine power plant effluents, particularly of flashed brine effluent, and have related to rapid scaling of reinjection equipment and wells. This is attributable to the fact that when hot geothermal brine (or other geothermal aqueous liquids) are flashed to a reduced pressure for extraction of steam, saturation levels in the brine of many scale-forming materials become exceeded. Instead of these scale-forming materials immediately precipitating, however, the brine tends to remain supersaturated, with the result that scale-forming precipitation occurs all along the subsequent brine flowpath. The resulting scale formation on exposed inner walls of downstream equipment and piping causes excessive maintenance requirements, and scale also tends to build up and choke-off the reinjection equipment and wells associated with the power plant, often at a rapid rate. The resultant reduction of brine flow through the power plant cuts power output and either necessitates periodic power plant shutdown for rework of the reinjection system or requires the costly provision of redundant reinjection facilities and wells.
Typically with silica-rich geothermal brines, representative of which are those found in the Imperial Valley of Southern California, silica scale buildup in the brine effluent reinjection system has often been so rapid that the associated power plants may need to be shut down for substantial periods every few weeks for reinjection system reconditioning. Not only is the scale, typically in the form of amorphous silicates, difficult, and hence expensive, to remove from reinjection equipment and piping, but costly drilling of new rejection wells may be required to replace badly scaled wells. In addition, costly power output interruptions result during associated power plant shutdown periods.
Prior art techniques for minimizing or inhibiting scaling, particularly silica scaling by silica-rich geothermal brines, have been successful only to varying degrees. In general, techniques which have shown some promise for reducing scale formation have heretofore been excessively costly. On the other hand, those which have been simple and inexpensive have heretofore usually been found to be relatively ineffectual at preventing downstream silica scaling. Thus, an important need exists for improved processes for treating silica-rich geothermal brines so as to prevent silica scaling, particularly in brine injection systems.
It is accordingly an object of the present invention to provide an effective, relatively inexpensive process for removing silica from silica-rich geothermal brines so as to substantially prevent downstream silica scaling.
It is another object of the present invention to provide an effective, economical and comparatively rapid silica removal process for stabilizing silica-rich geothermal brine so as to minimize brine residence time during the removal process.
It is still another object of the present invention to provide an effective, economical process for removing silica from silica-rich geothermal brine after flashing in a geothermal power plant so as to prevent silica-caused scaling in downstream brine effluent disposal portions of the power plant.
A yet further object of the present invention is to provide a silica removal process for stabilizing silica-rich geothermal brine which utilizes any indigenous ferrous iron dissolved in the brine in the removal process.
Additional objects, advantages and features of the invention will become apparent to those skilled in the art from the following description, when taken in conjunction with the accompanying drawings.